How do I mirror one side of an object to another side that's already there?
I'm going to provide 2 solutions to this answer: mirroring the side with the cone shaped holes, and secondly extruding that side to give bullet chambers that pass all the way through. Feel free to use either option.


Solution 1 - Mirrored Side Option




Delete the duplicated object Drum.001
Enter edit mode on the Drum object
Change selection mode to Edge and select an edge that connects both side of the cylinder.
Select S - Cursor to Selected (make sure you don't move the cursor after this step)
Select all the verts on the end that you are not keeping and delete.
Select all the verts on the other side that you want to mirror, and Shift D to duplicate
Press the . key to change the pivot type to Cursor
S Y -1 to scale along the y axis in the negative direction. This should copy your verts to the other side of the cylinder.
Select all, and Remove Doubles
Select all, and do Limited Dissolve (this is optional but it makes connecting the two side easier in the next step).
In edge select mode select the outer circle of both sides. Then SPACE and type Bridge to find the Bridge Edge Loops command.




Solution 2 - Extruded Side




Delete the duplicated object Drum.001
Enter edit mode on the Drum object
Change selection mode to Edge and select an edge that connects both side of the cylinder.
Select S - Cursor to Selected (make sure you don't move the cursor after this step)
Select all the verts on the end that you are not keeping and delete.
Select the tips of the cones and delete. You should now have one face with 6 holes in it.
Select all, and Remove Doubles
Select all, and do Limited Dissolve (this is optional but it makes connecting the two side easier in the next step).
Press the . key to change the pivot type to Cursor
With all verts select press E to extrude and Enter.
S Y -1 to scale along the y axis in the negative direction. This will move your extruded side to the other location giving you a full cylinder with 6 bullet chambers that pass all the way through.